The present invention disclosed herein relates to a solid immersion lens (hereinafter, referred to as ‘SIL’) holder for a high resolution optical microscope, and more particularly, to an optical axis precision correction SIL holder for a high resolution optical microscope which can allow the central axis (optical axis) of a SIL to maximally approach the central axis (optical axis) of an objective lens for correction while maintaining a close contact between the SIL and a specimen, so as to achieve a resolution beyond the diffraction limit of the objective lens, in an optical microscope for observing a specimen while coupling a hemispherical SIL having a hemispherical upper surface and a flat lower surface to the SIL holder having a plurality of slots which have a spring function.
Generally, the diffraction limit that denotes the limit of precision in regard to an infrared optical microscope among optical analysis measurement equipment using light is obtained by the following equation. The measurable resolution (d) is known as about 200 nm to about 300 nm.
  d  =            λ              2        ⁢        n        ⁢                                  ⁢        sin        ⁢                                  ⁢        θ              =          λ              2        ⁢                                  ⁢        NA            
Here, d is the measurable resolution, λ is the wavelength of incident ray, and NA is the numerical aperture.
The infrared optical microscope is provided with a plurality of functional lenses capable of accurately measuring the state of a specimen by applying a infrared ray to an object, and is provided with a hemispherical SIL having a top surface of a spherical shape and a bottom surface of a flat shape, which uses an evanescent wave, as shown in FIG. 1, so as to achieve a high resolution which was impossible due to the diffraction limit of light. Also, there is known a SIL optical microscope technology of interposing the SIL between an objective lens and a specimen and thus preventing a beam leakage due to mismatching of the refractive index.
The SIL optical microscope requires special conditions for a high resolution optical imaging work. The special conditions are that the bottom surface of the SIL and the measurement surface of a specimen need to be in close contact with each other less than about 100 nm, that the bottom surface of the SIL and the measurement surface of the specimen need to be maximally parallel to each other while forming horizontal surfaces, that the central axis (optical axis) of the objective lens and the central axis (optical axis) of the SIL maximally need to be in close proximity each other, and that the external forces acting on the specimen and the SIL need to be minimized.
That is, as shown in FIG. 2A, the bottom surface of the SIL and the measurement surface of the specimen may make contact with each other while spaced from each other by more than about 100 nm; as shown in FIG. 2B, the bottom surface of the SIL and the measurement surface of the specimen may not be parallel to each other; and as shown in FIG. 2C, the central axis (optical axis) of the objective lens and the central axis (optical axis) of the SIL may not be aligned with each other while not maximally approaching each other. In this case, the whole performance of the optical system may be reduced, and thus it is difficult to acquire a clear measurement image. Also, errors such as image overlapping and distortion may occur, and thus the reliability as a measuring apparatus for a high resolution imaging work cannot be secured.
Also, since the above-mentioned conditions are performed through an adjustment of about several nanometers to several micrometers upon setting of a high resolution optical measurement device including a SIL optical microscope, there are many difficulties in setting the measurement conditions of the optical measurement device.
Also, when a specimen is measured using the SIL optical microscope, as shown in FIG. 2C, the measurement surface may be slightly inclined. In this case, the SIL holder may be pressurized to adhere the SIL closely to measurement surface of the specimen, and thus one side of the SIL may make contact with one side of the measurement surface, thereby causing breakage of the SIL or the specimen.
Naturally, when the measurement surface of the specimen which slightly inclines is rightly positioned prior to the measurement, the above-mentioned limitation may not occur. However, in order to allow the specimen not to incline, the measurement surface and the opposite surface thereto need to be extremely precisely processed. Also, when an inclination surface is found during the measurement, the surface of the specimen needs to be reprocessed, causing a difficulty in the manufacturing of a specimen.
Accordingly, the SIL optical microscope desperately needs a structure capable of correcting the central axis (optical axis) of the SIL, capable of adjusting the focal length, and capable of easily maintaining the close horizontal contact state with the specimen even when the measurement surface of the specimen slightly inclines.
As a related art patent technology for easily maintaining the close horizontal contact of the SIL and the specimen in regard to the above-mentioned limitations of the SIL optical microscope, Korean Patent No. 10-1403992 owned by the present applicant discloses a device for fixing a near field lens. In this technology as shown in FIG. 3, a plurality of slits 101 that perform a spring function are formed at a side of a SIL mounting part 100 at a predetermined interval, so as to measure a specimen having an inclined measurement surface. Thus, a SIL holder is extended and contracted by the spring function, thereby allowing the inclined measurement surface and a SIL 102 to make close contact with each other.
As another patent technology of allowing a SIL and a specimen to make close contact with each other, U.S. Pat. No. 8,767,199 discloses an inspection system utilizing solid immersion lenses. In this technology as shown in FIG. 4, a flexure 115 that performs a spring function with a plurality of slots spaced at a predetermined intervals is formed in a SIL holder 130 mounted with a SIL 210, and thus the SIL 210 is adhered closely to an inclined measurement surface of a specimen by an extension and contraction of the flexure 115 of the SIL holder 130.
When the measurement surface of a specimen is inclined, as shown in FIG. 2C, the above-mentioned related-art patent technologies can incline the bottom surface of the SIL at the same angle as the inclination surface of the specimen by extending/contracting the plurality of slots, and thus can allow the bottom surface of the SIL and the measurement surface of the specimen to make close contact with each other. However, since the central axis (optical axis) of the SIL changes due to the inclination of the SIL and thus the central axis (optical axis) of the objective lens and the central axis (optical axis) of the SIL are not aligned with each other, the measured image becomes blurred, or a measurement defect such as image overlapping occurs. Accordingly, these technologies are inappropriate for high resolution measurement.
Since a typical SIL optical microscope enables low resolution measurement in regard to the measurement surface of an inclined specimen but the inclination of the specimen needs to be removed for high resolution measurement, excessive cost and effort according to the ultra precision process of the specimen are still needed.
As a related art for overcoming the limitation that the central axis (optical axis) of the objective lens and the central axis (optical axis) of the SIL are not aligned with each other, Korean Patent Application Publication No. 10-2008-0011814 discloses “optical head of apparatus for reproducing recording medium and method for manufacturing the same”. In this document, as shown in FIG. 5, an adhesive material 124 is injected to combine an objective lens 121 and a lens holder assembly 123 equipped with a SIL 122, and light reflected by a reflection mirror 125 is acquired by an image sensor. Thereafter, a tile component of the objective lens 121 is adjusted according to an interference pattern. In order to minutely adjust the tilt of the objective lens 121, a piezoelectric linear motor 126 is coupled to a side surface of the lens holder assembly 123, and then is driven before the adhesive material 124 is hardened, thereby controlling the location of the objective lens 121 according to the expansion and contraction of the piezoelectric linear motor 126.
However, since the related art of FIG. 5 is to control the location of the objective lens with the piezoelectric linear motor, the objective lens may be damaged as the driving force of the linear motor acts. Also, since the driving component is added, the structure is complicated, and a failure occurs frequently. In addition, there is another limitation in that electric power is needed for measurement.